1) Field of the Invention
The present invention relates to a polycarbonate resin with transparency, heat resistance, a low photoelastic constant and a good property balance between refractive index and Abbe's number, which is suitably applicable to plastic optical products including optical disc substrates, various lenses, prisms and optical fibers and a process for producing the same.
2) Prior Art
Polycarbonate resin from 2,2-bis(4-hydroxyphenyl)propane (the so-called bisphenol A) obtainable by reaction of bisphenol A with phosgene or carbonic acid diester is applied in a wide range to structure materials and optical disc substrates, various lenses, prisms and optical fibers as optical materials since it has excellent heat resistance, transparency and furthermore excellent mechanical properties such as impact resistance.
However, the polycarbonate resin from bisphenol A causes a problem in which double refraction becomes large due to molecular orientation or residual stress during molding since it is a material with low flowability and has a high photoelastic constant. Thus, when an optical material composed of the polycarbonate resin from bisphenol A is molded, a method for lowering double refraction of its product by molding at a high temperature using a polycarbonate resin with comparatively low molecular weight has been applied. However, since there is a limit for lowering double refraction of the polycarbonate resin produced from bisphenol A even in the above-mentioned method, a material with further low photoelastic constant and high flowability has been earnestly required in a partial field of optical material, particularly in the field of optical disc with recent expansion of use of optical material.
As a method for lowering a photoelastic constant of a polycarbonate resin, for example, as shown in Japanese Patent Kokai (Laid-open) No.64-66234, a method for copolymerizing bisphenol A with tricyclo (5.2.1.0.sup.2.multidot.6)decane dimethanol is known. However, the method causes deterioration of heat resistance and does not provide sufficient effects in lowering of photoelastic constant.
Further, as lens materials, particularly a spectacle lens material, CR-39 (diethyleneglycol bisarylcarbonate) has been widely used, but it causes a problem of low productivity since it is a thermosetting resin with a low refractive index of 1.50. Thus, it has been desired to solve such problem.